1. Field of the Invention:
The present invention relates to magnetometers, and more particularly, to magnetometers based on fiber optic interferometry.
2. Art Background:
As man ventures deeper into space and further explores his planet, the need to measure various physical parameters places increasing demands on state of the art measurement techniques. For example, the present plans to measure planetary, interplanetary and even intergalactic magnetic fields will challenge the capabilities of present day space magnetometers. A variety of methods are known for measuring magnetic fields, including magnetometers based on moving and stationary coils, Hall effect, thin films, flux gates, magnetic resonances, and super conducting devices. It is also known to use light carrying optical fibers for detecting a magnetic field. One method of detection involves passing a beam of polarized light through an optical fiber from one end to the other in the presence of a longitudinal magnetic field, and measuring the extent of rotation (twist) of the plane polarized light. The extent of rotation is dependent upon the prevailing magnetic field. (See for example, U.S. Pat. No. 3,936,742.) Direction of rotation depends upon the direction of the applied field. Using this "Faraday Effect" approach, only large currents and magnetic fields can be detected since the Verdet constant of most doped silica fiber is small. In addition, this approach requires special materials (i.e. silica fiber doped with rare earth ions to enhance the effect), and sophisticated fiber drawing techniques to provide reasonable magnetic field detection sensitivity.
Another approach which has been used in the past employs a Mach-Zehnder interferometer with one of the arms referred to as a sensor arm encoded or wound on a magnetostrictive material (MSM). When exposed to a magnetic field, the MSM undergoes dimensional change thereby altering the path of the beam traversing that fiber. The resulting phase difference between the two beams in the interferometer is directly related to the applied magnetic field. Using this technique, measurement sensitivities on the order of 10.sup.-[ 5.times.10.sup.-9 G/m of fiber has been reported. (See for example, U.S. Pat. No. 4,371,838.) However, due to hysteresis effects the response of the MSM to a magnetic field will depend on its previous magnetic history.
In another approach, a multimode optical fiber is used to detect electrical currents or magnetic fields from a remote source. The optical fiber is composited with metal capable of conducting electricity. Optical radiation is introduced into the fiber from a source which may either be coherent or incoherent. An electrical current is applied to the portion of the electrically conducting optical fiber, and the magnetic field is applied to the current carrying optical fiber. The stretching of the fiber in the presence of a magnetic field induces differential phase shifts in the light between the fiber modes. These phase shifts or losses are detected by a detector and the magnetic field strength thereby determined. (See U.S. Pat. No. 4,348,587.) However, this method does not permit direction or gradient measurement of the magnetic field.
As will be described, the present invention provides a fiber optic magnetometer which overcomes the above-referenced limitations in prior art magnetometers. The present invention employs a Mach-Zhender interferometer wherein one of the arms of the interferometer includes a metallic conductor attached to the fiber. The presence of a magnetic field is detected by the bowing of the fiber attached to the conductor through which a current is applied. The magnetic field direction may be determined from the current direction and fiber bend. The present invention provides a magnetometer which has been calculated to have sensitivity on the order of 10.sup.-18 Tesla/m. In addition, inasmuch as no ferromagnetic materials are used by the present invention, problems associated with hysteresis effects are avoided.